A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC--DC converters are frequently employed to advantage. DC--DC converters generally include an inverter, a transformer having a primary winding coupled to the inverter and a rectifier coupled to a secondary winding of the transformer. The inverter generally includes a switching device, such as a field-effect transistor (FET), that converts the DC input voltage to an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the DC--DC converter.
Conventionally, the rectifier includes passive rectifying devices, such as Schottky diodes, that conduct the load current only when forward-biased in response to the input waveform to the rectifier. Passive rectifying devices, however, cannot achieve forward voltage drops of less than about 0.35 volts, thereby substantially limiting a conversion efficiency of the DC--DC converter. To achieve an acceptable level of efficiency, DC--DC converters that provide low output voltages (e.g., 1 volt) often require rectifying devices that have forward voltage drops of less than about 0.1 volts. The DC--DC converters, therefore, generally use synchronous rectifiers. A synchronous rectifier replaces the passive rectifying devices of the conventional rectifier with rectifier switches, such as FETs or other controllable switches, that are periodically driven into conduction and non-conduction modes in synchronism with the periodic waveform of the AC voltage. The rectifier switches exhibit resistive-conductive properties and may thereby avoid the higher forward voltage drops inherent in the passive rectifying devices.
One difficulty with using a rectifier switch (e.g., an n-channel silicon FET) is the need to provide a drive signal that alternates between a positive voltage to drive the device into the conduction mode and a zero or negative voltage to drive the device into the non-conduction mode. Although a capacitive charge within the rectifier switch may only be 30 to 50 nanocoulombs, the rectifier switch requires a high drive current for a brief period of time to change conduction modes. Typical drive currents may be 10 amperes or greater, lasting for tens of nanoseconds. The need to provide substantial power to the rectifier switch to change conduction modes thus reduces some of the advantages of the synchronous rectifier.
The '138 patent, the '482 patent and the '541 patent all describe the use of the secondary winding of the transformer to directly drive the synchronous rectifier. The recognition of the availability of suitable drive voltages from the secondary winding over the entire switching cycle of the inverter led to the development of self-synchronized synchronous rectifiers as disclosed in the aforementioned patents.
The '032 patent describes the use of extra windings in the transformer and voltage-limiting switches to improve the control of the drive signal. The extra windings are particularly useful when the output voltage is so low that the secondary winding does not develop sufficient voltage to ensure that the rectifier switch is fully driven into the conduction mode. The voltage-limiting switches are useful when the input or output voltages are variable, resulting in wide voltage variations in the drive signal. The extra windings and voltage-limiting switches thus allow the transformer to provide drive signals of sufficient voltage to efficiently operate the synchronous rectifier.
When the switching frequency of a DC--DC converter is increased to achieve a more compact design, however, the energy required to charge and discharge the internal capacitance of the rectifier switch can result in substantial losses, detracting from, and ultimately limiting, the benefits ofthe low conduction mode resistance of the rectifier switch. As the duty cycle of the inverter changes to accommodate variations in either the load or the input or output voltage, wide variations in the voltage of the drive signal may result. Further, the transformer generates voltages of both positive and negative polarity, charging the control terminal of the rectifier switch to both positive and negative voltages. The variable nature of the drive signal detracts from the efficiency of the synchronous rectifier and presents an obstacle to increasing the switching frequency of the inverter.
Accordingly, what is needed in the art is a drive compensation circuit for driving the rectifier switch that avoids unnecessarily charging the control terminal of the rectifier switch to substantially negative voltages in the non-conduction mode and further avoids charging the control terminal ofthe rectifier switch to unnecessarily high positive voltages during the conduction mode, thereby increasing an efficiency of the synchronous rectifier. Additionally, in synchronous rectifiers employing at least two rectifier switches, a drive compensation circuit that equalizes the voltages applied to the control terminals of the rectifier switches may further increase the efficiency of the power converter.